Introduction

The introduction of laser scanners over the last few years adds another exciting technology that can be used on the manufacturing floor for combined safety and productivity. The puzzle a design engineer faces is when to use one in place of other established area protection technologies such as safety mats and light curtains.

Starting with the Safety Review

Area guarding design starts with the same process that is applied to safeguard any
machine - the safety review. If it is found that the addition of physical guarding is needed to lower risk, but methods such as mechanical barriers, interlocked fences, siding gates, and pull back restraints are too restrictive for operations, then area guarding technologies will be called upon.

To maintain safe operation, since there is no barrier from the hazard, the layout and size of the safety zone must be determined so that the machine can be safely stopped. The "safe minimum distance" takes into account how far the person can walk during the machine's stopping time.

How to Calculate the Safe Minimum Distance

The basis for the following safe minimum distance calculation is ANSI standard B11.19-2003. The ANSI formula consists of: Ds = K (Ts + Tc + Tr + Tspm) + Dpf

Also, some environments are too severe to allow personnel close to the operations. This would include areas that have dangerous debris being ejected, gases, heat, and even radiation. In such cases, a light beam or safety mat will not protect against the machine or its variant environment, specifically in regard to flying shrapnel and toxic, carcinogenic, or flammable materials.

Requirements for Application of Area Guarding Technologies

In general, if the application requires repetitious activity which, in itself, can cause operator fatigue and unsafe "work arounds" such as system bypasses, an area guard could be the right solution. Regardless of the technology selected, several important principles should be followed.

First, the safe distance must be correctly calculated and used in the design. If any modifications are made after the installation, the original safety measures should be
re-evaluated. It should be audited and maintained at predefined intervals with written procedures. You may also consider stating these in the operator's job description. Also, be aware that the stopping time could be extended due to machine wear, especially in the case of mechanical brakes.

Concerning the area protected, there should be no other points of entry that allow someone to be endangered. Since the protected area is invisible when using photo-electrics, a visible outline should be depicted. It could be either a painted line, rail, or chain. Another measure is to post a warning sign as well.

Lastly, the safety reset switch should be placed outside the zone and the operator must be able to see the complete area before starting the apparatus. For added safety, you may want to consider using a keyed switch to restrict this potentially dangerous activity further.

A Review of Each Electronic Area Guarding Technology

If your application fits the requirements of the last two sections, you can now evaluate which electronic area guarding technology will work best for you. In each case, we generally discuss the technology, installation details, state distinct benefits, and then suggest industrial fits.

Safety Mats

Since the 1950s, automatic door mats have used in numerous commercial and industrial applications around the world. Surprisingly, the first safety mats were not introduced until 30 years later for industrial safety. They are now a very practical solution for many applications.

A. General Technology

The safety mat is a simple, normally open switch. When a specified minimum weight
(66 pounds by ANSI B11.19-2003) is applied to the safety mat, the "switch" closes. This sends a signal to the safety mat controller which sends a stop signal to the guarded machine. If you have any malfunctions such as broken wires, shorted plates, the machine will not operate.

When the safety mat is not exposed to sufficient actuating force, the signals are unimpaired. The output relays in the controller are energized permitting the guarded machine to run. The mat itself is typically rated for more than a million actuations.

If you have a large area, you may want to interconnect multiple mats with joining trim. Mats are available in a wide variety of standard sizes. Custom shapes can frequently be made to order.

B. Installation details

Although mats are simplistic devices, they do require proper installation procedures. First, the surface on which the safety mat(s) will be placed should be flat, smooth and free of debris. After the mat is in place, you should use care in routing the mat cables to prevent damaging the insulation or cutting/shorting the internal wires. Lastly, you should perimeter trim is used to support (1.b) and tie multiple mats together.

C. Distinct Benefits

Safety mats have several key benefits that should be highlighted in their evaluation. For starters, they will work in many areas and are an alternative to light curtains in areas that may limit optical sensing such as mists, dust and smoke. Also, they are generally resistant to:

• Cutting oils
  
• Salts
  
• Welding splashes
  
• Shavings
  
• Grinding particles
  
• Water
  
• Mineral Acids
  
• Organic Acids
  
• Alcohols
  
• Aldehydes
  
• Caustics
  
• Petroleum products
  

Chemical resistance is dependent on factors such as time, temperature, and concentration. Confirm mat chemical resistance with manufacturer prior to purchase.

Once properly installed, mats require no maintenance other than the occasional sweeping and test procedure. They can also be used in conjunction with other safety equipment guarding a machine or work cell, such as opto-electric devices.

D. Industrial Fits

The resulting potential applications for safety mats include: